1. Field of the Invention
The present invention relates to methods of preparing a polymer composite using a giant magnetostrictive material, and more particularly, to polymer composites having various improved properties, in which the advantageous structure of the giant magnetostrictive material produced by unidirectional solidification can be maintained by removing the eutectic phase from the magnetostrictive material and filling resulting voids with a polymer resin.
2. Background Art
In general, the term “magnetostriction” refers to a Joule effect, where a material changes length in response to a change in magnetic field, and to its inverse effect, a Villari effect, where a magnetization state in the material changes in response to an external mechanical strain. Compared to PZT (lead zirconium tantanate) piezoceramic materials and nickel-based magnetostrictive materials conventionally used as transducers or actuators, rare earth element (RE)-transition metal magnetostrictive materials are higher in magnetostrictive strain, and have superior energy efficiency. Thus, the RE-transition metal magnetostrictive materials are superior for various applications, and, particularly, for aeronautics, communications, oil refining, automobiles, MEMS (Micro Electro Mechanical Systems) and medicine. More specifically, such materials are used for high output actuators, linear and rotary motors, control of noise and vibration, pumps, fuel injection, robotics, position determination, surveillance activities, valve actuators, micro-positioning, sonar, audio systems, ultrasonic instruments, force sensing, endurance measurement, etc.
The RE-transition metal magnetostrictive material, which is prepared by means of a unidirectional solidification, normally exists in the form of a single crystal or aligned poly-grains. In order to obtain high magnetostrictive strain in such an alloy, grains in the alloy must be arranged in a specific direction. This is because upon unidirectional alignment of the grains, the magnetostrictive strain increases, and internal loss generated at a grain boundary decreases. Such orientation in the magnetostrictive material is important for obtaining high magnetostrictive strains at low magnetic field.
A typical RE-transition metal magnetostrictive material having aligned grains is an alloy comprising Tb0.3Dy0.7Fe1.9. However, this alloy has the disadvantages of short span life and difficult processing, due to high brittleness of a Laves phase (REFe2 phase, in which “RE” refers to a complete solid solution of Tb and Dy). To solve these problems, larger amounts of terbium (Tb) and dysprosium (Dy) are added, or the iron (Fe) content is decreased, in order to form a larger amount of the eutectic phase having superior ductility and toughness at boundaries of the Laves phase. As such, the eutectic phase comprises a densely formed network structure throughout the material.
As the phase diagram of FIG. 4 shows, the eutectic phase comprises RE phase and REFe2 phase in a finely mixed state. When the total composition is very close to REFe2, only a small amount of the eutectic phase is formed. In this case, the REFe2 phase of the eutectic structure, which forms at the eutectic temperature, crystallizes at the nearby existing primary REFe2, and only the RE phase is formed between large REFe2 primary dendrites. This type of eutectic solidification is called “divorced eutectic solidification.” However, if the total composition is closer to the exact eutectic composition, a larger amount of the eutectic phase is formed in a regular type of finely mixed state.
The major disadvantage of the unidirectionally solidified RE-transition metal magnetostrictive material is its low electric resistance, attributable to the metal. Thus, since heat generation and energy loss are high due to eddy currents at high frequencies, the use of these materials is limited in applications requiring high frequencies. Hence, a method of reducing the eddy current loss is used that cuts the material into thin sheets (about 1 mm thickness) and layers them with an insulator in-between. However, such a layering process is difficult and expensive to perform, because the RE-transition metal magnetostrictive material has high magnetostrictive strain due to the maximized volume fraction of the intermetallic compound, and is therefore very brittle.
As an alternative, powders of the RE-transition metal magnetostrictive material are mixed with the polymer resin and prepared as a polymer composite, which is advantageous due to a simple preparation process, low cost and easy compaction to various shapes. However, since small grains having magnetostrictive property are dispersed in a non-magnetic polymer resin, such a composite is disadvantageous in light of inferior crystal orientation and low magnetization strength and thus much lower magnetostrictive strain, compared to the unidirectionally solidified material. Even so, more important advantages of the composite material are its good mechanical toughness and high electric resistance, due to the polymer matrix functioning as an electrical insulator. By increasing the electric resistance, heat generation due to eddy current is decreased, and the usable frequency range is increased from several tens of kHz to several hundreds of kHz. In the case of preparing the composite material with TbxDy1-xFe2-w, the total eddy current energy loss decreases to ⅙ compared to energy loss in unidirectionally solidified material. In addition, the composite material has higher toughness under tensile stress.
Conventionally, the polymer composite material is prepared as follows.
(1) The unidirectionally solidified RE-Fe magnetostrictive material ingot is pulverized in an inert gas atmosphere to make grains, so as to inhibit oxidation behavior thereof;
(2) The particles are mixed with a polymer binder and pores present in the grains are removed by evacuating;
(3) A mixture of the resin and the grains is compacted under pressure, while an external magnetic field of 100 Oe or more is applied perpendicularly relative to a pressure axis to the grains for their unidirectional alignment upon compacting. In this case, the external magnetic field is applied using an RE-cobalt (e.g., NdFeB) permanent magnet or a solenoid coil, whereby the grains forms a texture aligned in a magnetic field direction; and
(4) The polymer resin is cured to produce the composite having excellent mechanical properties.
At present, the polymer composite material is considerably lower in magnetostrictive strain than the unidirectionally solidified material. This is because the grains of the magnetostrictive material are very small and require a larger magnetic field upon application, due to the formation of the demagnetizing field in a reverse direction of the magnetic field to be applied, and also have poor crystal orientation. Fine grains are low in magnetostrictive strain because saturation magnetization strength of the surface is lower than saturation magnetization strength in the bulk. As the distance between grains increases, a coupling force between them is decreased. In particular, as in RE-iron magnetostrictive material, it is very important that the texture is maintained in the favorable crystal orientation, i.e., having a large magnetostriction.
The RE-transition metal magnetostrictive material can be further increased in its magnetic properties through annealing. As for the RE-iron magnetostrictive material, the annealing process is performed at a temperature higher than 887° C., the eutectic temperature (see FIG. 4). In some cases, a magnetic field may be applied during annealing. Although the magnetostrictive strain increases by alleviating the stress remaining upon solidification through the annealing, heat treatment effects are not great. Also, in the latter case, the stress transferring effects are drastically reduced due to the removal of the eutectic phase, thus the material exhibits low toughness.